1. Field of the Invention
Aspects of the present invention relates to an anode for a lithium rechargeable battery and a lithium rechargeable battery using the same, and more particularly, to an anode for a lithium rechargeable battery that uses a composite material containing metal, or non-metal, oxides thereof and carbonaceous material as an anode active material, and a lithium rechargeable battery using the same.
2. Description of the Related Art
Generally, as slimmed and lightweight portable electronic devices such as portable digital assistants (PDAs), cellular phones, notebook computers, digital cameras and others have been developed and widely used, batteries suitably used as a power supply of the portable electronic devices have been developed. Particularly, a lithium secondary battery is widely used in the electronic appliance field because the lithium secondary battery has a low self discharge rate and a high energy density per unit weight in comparison with other rechargeable batteries such as conventional lead batteries and nickel-cadmium batteries.
Lithium metal, which has a very high energy density, has been conventionally proposed as an anode active material of the lithium rechargeable battery. However, the use of lithium metal in a battery presents a safety problem in that dendrites are formed in the anode at charging, which can cause an internal short by penetrating into a separator. Moreover, the dendrites have a high reactivity because of a very large specific surface area, and react with the electrolyte of the battery, thereby forming a polymer film having low electron conductivity on the surface of the anode. Accordingly, the resistance of the battery is rapidly increased, or particles isolated from electron conduction network are generated, thereby interrupting charge/discharge.
Accordingly, graphite capable of absorbing and discharging lithium ions has been used as the anode active material instead of lithium metal. When graphite is used as the anode active material, the lithium metal is not deposited and dendrites are not formed. However, when the lithium metal is used for the anode, the theoretical discharge capacity is 3860 mAh/g, but when graphite is used for the anode, the theoretical discharge capacity is no more than 372 mAh/g. Accordingly, it would be desirable to have a new active material having a capacity higher than graphite.
Recently, materials forming compounds with lithium, that is, metals such as Sn, Al and Zn, non-metals such as Si, Ge, B and P, or oxides thereof have been proposed as the anode active material. An anode active material formed of one or more of these metals, non-metals or oxides thereof theoretically has a higher capacity than graphite, and accordingly, has a high initial capacity. However, there is a disadvantage that the capacity decreases rapidly after multiple charge/discharge cycles because the electrochemical reversibility of these materials is low. This disadvantage causes a reduction of the lifetime of the battery. Thus, a composite active material (hereinafter, referred to as “metal-carbon composite active material”) formed of the material forming compounds with lithium and a carbonaceous material, is proposed to solve the problem as described above.
The metal-carbon composite active material can be manufactured by embedding the metals such as Sn, Al and Zn, non-metals such as Si, Ge, B and P, or oxide particles thereof in the carbonaceous material, or coating them with carbon, or embedding them in the carbonaceous material and then mixing them at a high temperature.
The anode of the lithium ion battery basically includes an active material that participates in the electrochemical reaction of the battery, a collector, and a polymer binder that binds the active materials to each other and that fixes the active material to the collector. The active material used in the anode of the lithium ion battery is a particle type active material and fixed to the collector by the binder. The particles of the active material fixed by the binder are connected to the collector by being electrically coupled to each other by point contact. Accordingly, when the degree of point contact between the active material particles is low, in other words, when the point contact area is small, the internal resistance of the battery becomes high. Isolated particles that are not connected by point contact do not contribute to the capacity of the battery. Thus, it is desirable to maintain a large contact area between the active material particles.
On the other hand, during charging or discharging of the battery, lithium ions are inserted or released from the anode structure including the active material particles. The active material particles are expanded or contracted by insertion or release of the lithium ions. The amount of expansion or contraction differs according to the kind of the material used as the active material. For example, natural graphite undergoes a change of volume of a maximum of about 10%, whereas a metal-carbon composite active material undergoes a change of volume that is significantly more than the change of the natural graphite. Accordingly, the electrical coupling between the active material particles by point contact may become unstable according to the progression of charging/discharging.
The internal resistance of the lithium ion battery is gradually increased and the capacity is gradually decreased at every charging/discharging because of the volume change and the instability of contact between the active material particles according to charging/discharging. As a result, the lifetime of the battery is shortened. The shortening of the lifetime caused by the volume change of the active material may be somewhat improved by adding a material such as carbon black as a conductive material. However, an excessive amount of conductive material causes a decrease in the proportion of the active material, thereby reducing the discharge capacity of the battery.